Biogas stripping of ammonia from fresh digestate from a food waste digester

• Biogas stripping reduced ammonia concentrations of fresh SS-DFW digestate.
• Ammonia removal from fresh digestate was more difficult than from stored digestate.
• Hydrolysis was improved by thermal-alkaline stripping.
• In situ ammonia removal is not feasible at mesophilic or thermophilic conditions.
• Ammonia stripping was most effective at 70 °C and pH 10.

The efficiency of ammonia removal from fresh source-segregated domestic food waste digestate using biogas as a stripping agent was studied in batch experiments at 35, 55 and 70 °C, at gas flow rates of 0.125 and 0.250 Lbiogas min−1 L−1digestate with and without pH adjustment. Higher temperatures and alkaline conditions were required for effective ammonia removal, and at 35 °C with or without pH adjustment or 55 °C with unadjusted pH there was little or no removal. Results were compared to those from earlier studies with digestate that had been stored prior to stripping and showed that ammonia removal from fresh digestate was more difficult, with time constants 1.6–5.7 times higher than those previously reported. This has implications for the design of large-scale systems where continuous stripping of fresh digestate is likely to be the normal operating mode. A mass balance approach showed that thermal-alkaline stripping improved hydrolysis.